The in-line measuring device includes a measurement pickup, especially a magneto-inductive measurement pickup, having a measuring tube for conveying the fluid to be measured, lined internally with a liner and inserted into the course of the pipeline, and wherein the liner is made of a polyurethane produced using a catalyst containing metal-organic compounds.
An electric voltage induced in the fluid is, therefore, measurable by means of at least two measurement electrodes arranged spaced from each other in the direction of the electric field, and by means of an evaluation electronics of the in-line measuring device connected to these electrodes. The induced voltage is, in turn, a measure for the volume flow rate. Fluid-contacting, galvanic, or fluid-not-contacting, capacitive, measurement electrodes can, for instance, serve to sense the induced voltage. For conveying and coupling the magnetic field into the measurement volume, the magnetic circuit arrangement generally includes coil cores surrounded by the field coils. The coil cores are separated from each other especially diametrically along a periphery of the measuring tube, and are arranged with, in each case, a free end face front surface essentially facing the other, especially at positions where they are, in effect, mirror images of one another. In operation, the magnetic field created by the field coils connected to the exciter-electronics is so coupled via the coil cores into the measurement tube, that it passes through the fluid flowing between the two end faces at least sectionally perpendicularly to the stream direction.
In-line measuring devices that measure flow velocities and/or volume flow rates of flowing fluids acoustically by means of ultrasonics are often used as an alternative to in-line measuring devices with magneto-inductive measurement pickups.
Due to the high mechanical stability demanded for such measuring tubes, these—both for magneto-inductive, as well as for acoustically measuring, measurement pickups—comprise mostly an outer, especially metallic, support tube of predetermined strength and diameter, coated internally with an electrically non-conductive, insulating material of predetermined thickness, the so-called liner.
For example, U.S. Pat. No. 6,595,069, U.S. Pat No. 5,664,315, U.S. Pat. No. 5,280,727, U.S. Pat No. 4,679,442, U.S. Pat. No. 4,253,340, U.S. Pat. No. 3,213,685 or JP-Y 53-51 181 each describes magneto-inductive measurement pickups, which include a measuring tube insertable fluid-tightly into a pipeline. The measuring tube, which has a first, inlet end and a second, outlet end, is comprised of a non-ferromagnetic support tube, serving as an outer casing of the measuring tube, and a tubular liner accommodated in a lumen of the support tube. The liner, which is made of an insulating material, serves to convey a flowing liquid insulated from the support tube.
The liner, which usually is made of a thermoplastic, thermosetting or elastomeric, synthetic material, serves to chemically isolate the support tube from the fluid. In the case of magneto-inductive measurement pickups, whose support tube has a high electrical conductivity, for example through the use of metallic support tubes, the liner serves also as electrical isolation, or insulation, between the support tube and the fluid, that prevents a short circuiting of the electrical field through the support tube. By suitable design of the support tube, it is thus possible to match the strength of the measuring tube to the mechanical loads in particular cases of application, while an adapting of the measuring tube to the chemical and/or biological requirements of particular applications can be realized by means of the liner.
Because of its good workability on the one hand, and its good chemical and mechanical properties on the other hand, polyurethane, in particular, has, alongside hard rubber or fluorine-containing synthetic materials such as PTFE, PFA, also become established as material for liners of in-line measuring devices, especially those with magneto-inductive measurement pickups. Furthermore, liners of polyurethane have mostly good biological properties, especially also in bacteriological regard, and are to that extent also suitable for application to aqueous fluids.
The polyurethanes used for the production of the described liners are mostly elastomeric plastics, that are made on the basis of liquid, multicomponent systems formed, directly before the processing, of reactive starting components. Following mixing, such multicomponent system is applied onto the adhesive-agent-pretreated, inner wall of the support tube and left there to cure to form the liner within a predeterminable reaction time. It is well known that polyurethanes are made by the polyaddition method from di- and poly-isocyanates and di- or polyvalent alcohols. The starting components can, in such case, be, for example, prepolymers, composed of aliphatic and/or aromatic ether-groups, as well as glycol-, and isocyanate-, groups. Such prepolymers then react with the supplied, di- or polyvalent alcohol.
Often used to manufacture liners of polyurethane is a so-called ribbon flow method, in which the previously prepared, liquid, multicomponent system is evenly distributed on the suitably moving, inner wall of the support tube by an appropriate pour-, or spray-, head. The reaction time required for the subsequent curing of the multicomponent system can be set, by the dosage of the starting components, also to a large extent by a suitable controlling of the processing temperature. However, short reaction times of less than a minute, which are necessary for cost-effective production of the liner, at a processing temperature of about room temperature, are obtained usually only through addition of a suitable catalyst, usually heavy metal and/or amine-containing, to the multicomponent system. Here, especially tertiary amines and/or mercury are used as catalysts. Considering that the catalyst itself remains essentially unchanged in the finished polyurethane, the latter has to this extent inevitably also toxic, or at least physiologically not completely harmless, characteristics. Numerous investigations have also shown, that especially the catalyst can, to a significant degree, be dissolved out of the liner at least in the presence of water. To that extent, the polyurethanes used at present in in-line measuring devices are only suitable conditionally for applications with high hygienic requirements, e.g. for measurements in the field of drinking water, since the high demands for fluid-touching components in the drinking water field with regard to chemical stability as well as physiological compatibility, cannot, without more, be fulfilled. In the drinking water field, special attention is paid among other things to the adherence to the maximally tolerable migration rate (Mmax,TOC) with regard to total organic carbon (TOC) content and/or the specific migration limit (SML) values defined for toxicologically critical substances. Equally strict are the requirements regarding the effect of the liner on the aesthetic condition of drinking water, especially regarding taste, color, turbidity, and/or smell neutrality of the liner in the presence of water, as well as regarding the maximally tolerable chlorine consumption rates (Mmax,Cl).